On the other hand, after a small spark has created a deflagration, the transition process can take several meters or longer and a corresponding large amount of time. The key to detonation initiation schemes applicable topulse detonation engines is to significantly shorten the distance and time required for deflagration-to-deto-nation transition.

Deflagration-to-detonation transition is the general process by which a subsonic combustion wave(deflagration or flame) becomes a supersonic combustion wave (detonation

So I guess that means, don't have too long of a chamber (like a few meters long.)